docs: def add_check(self, check):
      """
      Please use rather `register_check` as a decorator.
      """
      if self._add_check_callback is not None:
        if not self._add_check_callback(self, check):
          # rejected, skip!
          return False
  
      self._checkid2index[check.id] = len(self._checks)
      self._checks.append(check)
      return True